组成细胞的主要元素是____ 　、____ 、____ 、____ 、____ 、____ ．细胞中的元素绝大多数以____ 形式存在，其中含量最多的有机化合物____ ．

如图甲所示，一轻质弹簧的下端固定在水平面上，上端放置一物体（物体与弹簧不连接），初始时物体处于静止状态．现用竖直向上的拉力F作用在物体上，使物体开始向上做匀加速运动，拉力F与物体位移x之间的关系如图乙所示（g=10m/s²），则下列结论正确的是（　　）

阅读短文，判断正确还是错误。

A letter

Here is a letter from Jim to his friend in the USA

Dear Lily:

    I am in China now. I go to school five days a week. The school and the food are good here. I have some Chinese friends. They are all very friendly to me. On Saturdays and Sundays we stay at home. In the afternoon we play games. My Chinese friends like English very much. They ask me to teach them English and they teach me Chinese. I like staying here.

    Please write soon!

（2014.宁波）如图是研究磁体周围磁场时的铁屑分布情况。实验时，a、b、c三个位置所对应的磁极可能是（    ）